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MANCHESTER — A flash fire inside an underground fuel storage tank severely burned two South Carolina men who were relining the 10,000-gallon tank with fiberglass early Monday afternoon.

The men — ages 24 and 33 — were flown by medical helicopter to Massachusetts General Hospital in Boston with life-threatening, third-degree burns, fire officials said. Their conditions are critical, officials said. Their names were withheld until family is notified.

Both men were part of a crew of five from Tank Tech of Blodgett, Mo., hired to refurbish the diesel bay's underground fuel tank at the Mobil Kwik Stop at 1095 Hanover St.

A work light hanging from the man way above the tank somehow dislodged about 1 p.m., fell in the tank and broke, igniting the fire with a spark of electricity, Fire Chief James Burkush said.

One worker escaped the 8-foot-deep tank on his own, despite being severely burned.

"I saw him crawl out and roll over on his back," said John Brewer, who runs the Citgo station across the street, Johnny B's Car Care. "I went to move him. He said, 'Don't move me! My legs!' I looked down. I thought his pants were just ripped. But they were melted off."

Brewer, 42, of Lowell, Mass., told employees inside the Kwik Stop to call 911, then went outside when he learned a second man still was inside the tank.

A co-worker entered the tank and placed a harness on the wounded man, attaching it to a cable which Brewer said he and co-workers used to pull the burned man out by hand.

"He was melted from head to toe," Brewer said.

"It was all melted fiberglass that was on the guy," he added, noting he was breathing when pulled from the tank.

Burkush said the two men had relined the interior of the tank with a fiberglass pad, then spray-coated a substance over it that would cause it to harden.

"The hardening hadn't happened yet and it was still a flammable atmosphere," Burkush said.

A supervisor standing outside the hole quickly doused the fire with two fire extinguishers. One of the men on the crew is the brother of one of the fire victims.

"This crew is quite experienced. It appears to be an unfortunate accident," Burkush said.

The fire remains under investigation by the Manchester firefighters, the state Fire Marshal and the U.S. Occupational Safety and Health Administration.

Fire investigators went into the tank to collect evidence to document how the fire started.

The fire did not disrupt classes at Weston Elementary School, located diagonally across Hanover Street.

"There was a very large black cloud of smoke that I saw from my office window," Principal Liz MacDonald said. Some smoke entered the school through the ventilation system.

The service station and the Citgo station across the street are owned by Aranosian Oil Company Inc. of Manchester. The company owns about a half-dozen gas stations in the city. John Aranosian could not be reached for comment.